1. Field of the Invention
This invention relates to a radio receiver, and is applicable for example, to a digital cellular system transmitting and receiving voice signals by coding them.
2. Description of the Related Art
Conventionally, a digital cellular system, which is a kind of radiotelephony, is designed to allow a plurality of terminals to simultaneously use one channel through a time division multiplex approach by coding voice signals for transmitting and receiving.
That is, when it is turned on, such a type of terminal sequentially scans received frequencies in a predetermined number of channels, for example, 124 channels, and detects a FCCH (frequency correction channel) which is inserted in the channel at a predetermined cycle (typically, consisting of 10 or 11 frames) in order stronger field strength, to recognize the channel including the FCCH as control channel.
Then, the terminal detects and receives a control channel assigned to the area to which it belongs.
The control channel is designed to form a time slot for transmission of various information, whereby, in the digital cellular system, each terminal receives the control channel to receive information such as information on a base station sending the control channel, information on an adjacent base station, and information for calling the terminal.
To this end, the terminal corrects the processing timing based on this FCCH.
Here, the FCCH is a synchronizing signal assigned with a bit pattern in which, when it is decoded, data of value "0" continues for a predetermined number of bits. In the digital cellular system, the bit pattern is differential coded and then GMSK (Gaussian filtered minimum shift keying) modulated for transmission.
Thus, the digital cellular system detects this FCCH, and roughly synchronizes the entire operation based on the result of this detection (that is, frame synchronization).
At the completion of frame synchronization, such a type of terminal corrects the deviation of frequency of its internal clock to the base station based on the FCCH, and then finely synchronizes the entire operation based on the subsequent predetermined reference signal.
When a state ready for receiving the control channel is thus established, the terminal receives control data sent from the base station by receiving the time slot assigned to it, and switches the transmission/receiving frequency from the control channel to a call channel, as required.
Then, the terminal transmits or receives a voice signal to or from a called station by using this call channel.
At the moment, the digital cellular system is designed to add an error correction code to the data to be transmitted, and sends out the data after performing differential coding and GMSK modulation, whereby stable communication can be assured.
Such a type of terminal may be used on a car or the like. In such a case, the terminal moves at a high speed in respect to the base station.
In this case, assuming that the relative moving speed between the terminal station and the base station is "v", and the transmission carrier frequency of the digital cellular is ".lambda.", the transmitted/received signal is doppler shifted by a frequency f.sub.dp as represented by the following equation: EQU f.sub.dp =v/.lambda. (1)
As shown in FIG. 1, if the mobile station with a terminal on board is moving in respect to the base station with an angle .alpha., the equation (1) is represented by: ##EQU1##
In this case, because the carrier frequency is selected for about 950 MHz in the digital cellular system, when the terminal moves at 250 km per hour with respect to the base station, this relationship is substituted for equation (2), thereby finding that the received signal is doppler shifted in frequency by 220 Hz at the maximum.
Such a doppler frequency shift is observed at the terminal as a deviation of the carrier frequency, resulting in a reduction of the decoding efficiency when demodulating the data and deterioration of the bit error rate for the entire terminal.
In such a case, an approach may be possible to correct the doppler shifted carrier frequency based on the above-mentioned FCCH. However, because the amount of the doppler shift varies depending on the moving speed of the terminal, the doppler shift may not be properly corrected in correspondence with the variation in the amount of shift if based on the FCCH which is intermittently inserted in the control channel.
Particularly, when the terminal moves as in the above, fading may often be caused as well. In such a case, the level and phase of the received signal simultaneously vary so that the correction of doppler shift becomes difficult.
On the other hand, a method has been proposed in which received data is stored once in a predetermined storage means, the amount of frequency shift being estimated based on the stored data, the frequency shift being corrected with the result of the estimate (The Transactions of IECE Japan, B-11, Vol.J73-8-11, No. 11, pp. 736-744, November 1990). This method causes the entire configuration to be complicated and consequently has such a problem that it can be applied to a base station, but is difficult to be applied to a terminal.
For this reason, in such a digital cellular system, the drift of the carrier frequency is specified as being 0.1 ppm or less (equivalent to a frequency of 95 Hz for the carrier frequency of 950 Hz. If the frequency doppler shifts as much as 220 Hz as in the above, not only is the bit error rate deteriorated, but it also becomes difficult to attain synchronization with the base station.
In addition, such a digital cellular system has a characteristic that the effect of doppler frequency shift becomes significant when external noise is low, in which there arises such a strange situation that the reception state is poor although sufficient field strength can be assured.